Normally tacky pressure-sensitive adhesive (hereinafter referred to by the abbreviation "PSA") compositions suitable, for example, for use in adhesive tapes must have an art-recognized.sup.1 four-fold balance of adhesion, cohesion, stretchiness and elasticity. PSA coated tapes have been produced and sold for at least a half century. FNT .sup.1 1952 Fall Symposium, Division of Paint, Varnish and Plastics Chemistry, American Chemical Society.
The early PSA tapes relied upon natural rubber for the elastomeric base and wood rosins as tackifiers to provide adhesive compositions with the requisite four-fold balance of properties. While tackified natural rubber provided a PSA composition which was of commercial significance, improvements in such compositions were sought because of the expanded expectation level of performance of PSA compositions. Various improved PSA compositions were thus developed.
Ionic polymerization produced block copolymer elastomers such as linear AB and ABA block copolymers which were likely candidates for the elastomer base in the PSA compositions and many were incorporated into such compositions to produce adhesives having high performance characteristics. For example, Harlan (U.S. Pat. No. 3,239,478) produced PSA compositions based on ABA block copolymer, tackifier resin and extender oil, recognizing that improved tack and cohesive strength could be obtained despite a heavy loading of extender oil. Miller (U.S. Pat. No. 3,519,585) produced an improved PSA composition having high peel strength, creep resistance and tack by blending AB and ABA block copolymers with a tackifier resin.
Other elastomer candidates for preparing PSA compositions include so-called branched block copolymers, radial teleblock copolymers and multiarm star block copolymers. The various polymer structures described by the term "branched", "radial" and "star" are not the same. "Branched" is a generic term indicating a nonlinear structure which may contain various polymeric subunits appended to various places on a main polymer chain or backbone. Such structures are typically complex in nature and may be derived by free radical, cationic or anionic polymerization. The term "radial" generally refers to branched polymer structures obtained by linking individual polymeric segments to yield a mixture of polymers having four or fewer arms joined centrally. The term "star" describes the structure of a multiarm polymer with copolymer arms which are joined together at a nucleus formed of a linking group which is virtually a point relative to the overall size of the remainder of the polymer structure. Non-terminating coupling agents, those in which the polymerizing anionic structure is retained, are generally preferred as linking agents for "star" structures.
While several references disclose preparing adhesive compositions or PSA compositions employing branched block copolymers, radial teleblock copolymers and multiarm star block copolymers, none recognize that a unique combination of superior shear holding power and desirable melt viscosity may be obtained by selecting only star block copolymers having an average of 12 or more arms to prepare a tackified adhesive composition. For example, Fetters et al (U.S. Pat. No. 3,985,830) recognize that adhesives may be prepared from star polymers but fail to specify details of the composition of the same or that the same is a PSA. St. Clair (U.S. Pat. No. 4,444,953) describes asymmetric star block polymer having at least 6 arms prepared by terminally linking together a mixture of styrene-isoprene AB block polymers and isoprene homopolymers. St Clair's disclosure compares the physical properties of a conventional star copolymer having an average of 8.6 arms with the asymmetric star polymers and of adhesives made therewith. Such comparison shows these polymers possess a relatively lower tensile strength and, when formulated as an adhesive, a relatively high melt viscosity, which may indicate to one skilled in the adhesive art that adhesive compositions containing such star polymers may not have superior shear strength properties and may not be easily melt processable. Hsieh et al. (U.S. Pat. No. 4,136,137) discloses the preparation of adhesive compositions including so-called branched radial polymers which have between three or four branches or arms. Marrs et al. (U.S. Pat. No. 3,658,740) discloses the preparation of PSA compositions by combining branched block copolymers which may have as few as 3 branches with linear block copolYmers, tackifiers and organic solvents. Marrs designed a PSA formulation which requires a tackified blend of linear and multiarm block copolymers employing a solvent as a critical element in the resultant adhesive formulation which was designed to bond to a wide variety of substrates but fails to address the need for hot melt processability. Nash (U.S. Pat. No. 4,148,771) discloses the preparation of PSA compositions employing rubbery coupled conjugated diene/monovinyl-substituted aromatic hydrocarbon teleblock copolymers. Nash attacks the problem of improved holding power for PSA by incrementally charging the styrene and initiator prior to addition of the butadiene in the preparation of copolymer to produce a PSA composition. While the resulting PSA may have improved holding power, it would not have a lower melt viscosity or ease of processing. Feeney (U.S. Pat. No. 4,288,567) employs a branched block copolymer described in Prudence (U.S. Pat. No. 3,949,020) then relies upon a solution preparation process to achieve an adhesive composition having increased tack, faster molten solution time and improved tack retention in hot melt blends. Feeney requires a compatible tackifying resin be combined with the block copolymer in its polymerizate form but fails to consider the need for ease of processing a hot melt of his adhesive composition. Hansen (U.S. Pat. No. 4,133,731) modifies star block copolymer PSA by using electron beam or ultraviolet radiation in combination with a supplemental crosslink-promoting tetrafunctional acrylate to improve holding power but not ease of processing of his adhesive composition.
The aforementioned prior art fails to recognize that the shear strength of the PSA may be optimized without sacrificing the ease of processing. Applicants' adhesive composition provides an adhesive composition with improved shear strength and good processability which can be used in packaging tape applications where shear holding power is necessary.